Projection optical apparatus using plural wavelengths of light

ABSTRACT

A projection optical apparatus comprising a projection optical system for projectively focusing a pattern image of a mask under illumination by light of first wavelength onto a sensitive substrate, a stage holding the sensitive substrate, a fiducial plate disposed on the stage, a first mark detector for illuminating light of second wavelength different from the first wavelength, through a first mark area formed on the mask and the projection optical system, onto .[.the sensitive substrate or.]. a second mark area formed on the fiducial plate, then detecting optical information produced from the second mark area, a fourth mark area formed on .[.the sensitive substrate or.]. the fiducial plate and arranged in a predetermined positional relationship relative to the second mark area, a third mark area formed on the mask and arranged in a predetermined positional relationship relative to the first mark area, a second mark detector for illuminating the light of first wavelength onto the fourth mark area through the third mark area and the projection optical system, and then detecting optical information produced from the fourth mark area, under a condition that the first mark detector is detecting the optical information produced from the second mark area, and an error detector for detecting detection errors due to a distortion at respective positions in the view field of the projection optical system where the first mark area and the second mark area are present, based on the detected results by the first mark detector and the second mark detector.

.Iadd.This is a continuation of reissue application Ser. No. 08/571,631filed Dec. 13, 1995, which is a continuation of reissue application Ser.No. 08/288,767 filed Aug. 11, 1994, both now abandoned..Iaddend.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a projection exposure apparatus used inthe lithography process for semiconductor devices, liquid displaydevices or the like, and more particularly to an alignment apparatuswhich can reduce an extent of alignment errors attributable todistortion caused by a projection system of the projection exposureapparatus.

2. Related Background Art

Recently, steppers mounting projection lenses of large numericalaperture thereon have been widely used as apparatus for printing apattern of masks (reticles) on semiconductor wafers with resolution onthe order of submicrons.

In such a stepper, a chip pattern (shot area) already formed on thewafer and a reticle pattern newly exposed in superposed relation must bealigned with each other at overall precision less than a fraction of theminimum line width. Of late, therefore, steppers mounting alignmentapparatus (sensors) with an ability of higher precision thereon havebeen researched and developed for practical use.

In the future, it is believed that those steppers which will beprimarily used in producing 4 Mbit D-RAMs and 16 Mbit D-RAMs will ofnecessity have a die-by-die exposure mode using the TTR(Through-The-Reticle) method in which a mark on the reticle and a markin each shot area on the wafer are successively detected and alignedwith each other, followed by printing.

While various techniques have so far been proposed to implement the TTRalignment method, most promising one is a different wavelength TTRalignment method in which the reticle mark and the wafer mark aresimultaneously detected by using an illumination light different inwavelength from an exposure light. This type alignment method isadvantageous in that because there will not occur a phenomenon for aresist layer on the wafer to strongly absorb the exposure light, themark can be stably detected even for such wafers as having a resistimpregnated with dyes or a multi-layered resist, and the resist in amark area can be prevented from being exposed or sensitized uponillumination for alignment. Typical techniques (projection exposureapparatus) well known as to implement the different wavelength TTRalignment method are disclosed in U.S. Pat. Nos. 4,251,160, 4,269,505,4,492,459 or 4,473,293.

In any of those typical projection exposure apparatus, however, anoptical system for correcting chromatic aberration of the illuminationlight having the different wavelength for alignment is disposed betweena reticle and a projection lens. Such a correction optical system servesto maintain the reticle mark and the wafer mark in focused relation toeach other under the illumination light having the different wavelength,but has suffered from an intrinsic problem that stability isinsufficient and precise reproducibility cannot be obtained in thealignment.

In the last several years, a method permitting the different wavelengthTTR alignment without using such .[.an.]. .Iadd.a .Iaddend.correctionoptical system with less stability and reproducibility has been proposedin U.S. Pat. No. 4,880,310 or Japanese Patent Laid Open No. 63-283129(corresponding to U.S. application Ser. No. 192,784 filed on May 10,1988). In an alignment system disclosed in the above reference, beamsfor illuminating the reticle mark and the wafer mark are simultaneouslyfocused by a two-focusing element and an object lens on two planes,respectively. One plane is coincident with a pattern plane (mark plane)of the reticle, whereas the other .[.plate.]. .Iadd.plane .Iaddend.iscoincident with a wafer conjugate plane in a space away from the reticlepattern plane by a distance corresponding to the amount of axial(on-axis) chromatic aberration of the projection lens.

Adopting the above disclosed method eliminates the need of providing anoptical element, other than the projection lens, in an optical path foralignment between the reticle and the wafer, and permits the TTRalignment as if the exposure light is used.

However, the latest projection lens is corrected in its variousaberrations satisfactorily for only the exposure light, but exhibitsboth axial chromatic aberration and magnification chromatic aberration.Even if use of the two-focusing element succeeds in correcting the axialchromatic aberration, the magnification chromatic aberration cannot bealways corrected satisfactorily, thereby requiring it to remove analignment error (offset) attributable to the magnification chromaticaberration by some method. For this reason, a technique of coping withthe magnification chromatic aberration (distortion) has been proposed inU.S. Pat. Nos. 4,780,913 and 4,856,905, by way of example. Of them, U.S.Pat. No. 4,780,913 discloses such a technique that a reference markcapable of exiting light rays at two wavelengths, i.e., the exposurelight at one wavelength and the illumination light for alignment at adifferent wavelength, is provided on a wafer stage and moved on theimage plane side of a projection lens to scan a retroprojected image ofthe reference mark on the reticle side for determining a position of thereticle mark, thereby preparing a distortion map in the view field ofthe projection lens beforehand. On the other hand, U.S. Pat. No.4,856,905 discloses such a technique that the exposure light isintroduced in the form of a beam to an illumination beam transmittingsystem (comprising a scanner mirror, two-focusing element, object lens,etc.) of a different wavelength TTR alignment system, allowing anillumination beam at the different wavelength and an illumination beamat the wavelength of the exposure light to be scanned simultaneously,whereby data of light information are photoelectrically detected fromrespective reference marks on the reticle mark and the wafer stage todetermine a distortion at the alignment point based on differences inthe amount of position deviation detected for each wavelength of thebeam.

The aforementioned prior arts for coping with the magnificationchromatic aberration are arranged so that both the illumination lightfor alignment and the illumination light for exposure pass through thesame position in the view field of the projection lens. Specifically, inU.S. Pat. No. 4,780,913, two types of illumination light are required tobe introduced to the rear side of the same reference mark throughoptical fibers or the like. In U.S. Pat. No. 4,856,905, the illuminationlight for exposure is required to be introduced to the differentwavelength TTR alignment system.

That structure has suffered from problems as follows. An arrangement ofthe alignment optical system is complicated, which leads to a difficultyin manufacture that a severer level is required in the performance ofconstituent optical elements (particularly, achromatism). In addition,it is difficult to make a match between various conditions of theoptical system (such as a sigma value, number of aperture andtele-centricity) under the illumination light for exposure and variousconditions of the optical system under the illumination light foralignment, thus rendering it hard to know a precise distortion errorenough for practical use. Moreover, an actual exposure apparatus(stepper) of even 1/5 reduction type is designed so as to accommodate anexposure of a wide field (view field) on the order of 15×15 mm to 20×20mm. But, exposure areas of reticle patterns employed by stepper usersare versatile in size, and the position of the alignment mark in theprojection field is naturally changed variously depending on the size.Because the distortion amount of the projection lens with respect to anideal lattice under the illumination light for exposure is also changeddepending on change in the alignment position, there has been anotherproblem that the difference between distortion characteristics under theillumination light for alignment and the illumination light forexposure, which has been determined at only the alignment position isnot enough for satisfactory correction, taking into account the factthat the reticle pattern must be superposed with the shot area over theentire wide field.

Further, the apparatus disclosed in the above cited U.S. Pat. No.4,856,905 has a specific problem as follows. Where the illuminationlight for exposure and the illumination light for alignment areseparated from each other depending on their ranges of wavelength by adichroic mirror obliquely disposed above the reticle at an angle of 45°,if transmissivity (or reflectivity) of the dichroic mirror for theillumination light for alignment is set very high, the illuminationlight for exposure to be detected by the different wavelength TTRalignment system could not pass through (or be reflected by) the TTRalignment system in its large part, making is difficult to detect themark. On the other hand, if wavelength characteristics of the dichroicmirror are selected to give some degree of transmissivity (orreflectivity) to the illumination light for exposure as well, the amountor intensity of light directing toward the reticle from an exposurelight illuminating system during the exposure would now be reducedcorrespondingly.

SUMMARY OF THE INVENTION

The present invention has been achieved in view of the various problemsas stated above, and has for its object to provide a projection opticalapparatus equipped with a different wavelength TTR alignment systemwhich can detect and correct an alignment offset attributable to,particularly, a distortion with high precision.

To achieve the above .[.problem.]. .Iadd.object.Iaddend., with thepresent invention, a fiducial mark (fourth mark area) to be detectedunder an illumination light for exposure (light of first wavelengthcharacteristic) and a fiducial mark (second mark area) to be detectedunder an illumination light for alignment (a light of second wavelengthcharacteristic) are arranged on a fiducial plate to be positionallyseparate from each other, the fiducial plate being fixed on a stage onwhich a sensitive plate such as a wafer is rested. Furthermore, areticle mark (third mark area) to be detected under the illuminationlight for exposure and a reticle mark (first mark area) to be detectedunder the illumination light for alignment are disposed on a reticle(mask) respectively corresponding to the separate positions of theassociated fiducial marks on the fiducial plate. A different wavelengthTTR alignment system (first mark detection means) for detecting thereticle mark (first mark area) and the fiducial mark (second mark area)under the illumination light for alignment, and an exposure wavelengthTTR alignment system (second mark detection means) for detecting thereticle mark (third mark area) and the fiducial mark (fourth mark area)under the illumination light for exposure are arranged as independentoptical systems of each other and to be capable of detecting thecorresponding marks simultaneously by both the TTR alignment systems.

FIG. 1 shows a schematic configuration of a stepper for explaining theprinciples of the present invention. A circuit pattern area PA on areticle R is projected to one shot area on a wafer (not shown in FIG. 1)by a projection lens PL which is tele-centric on both sides. Around thepattern area PA on the reticle R, there are provided marks Au, Al to berespectively detected by TTR alignment systems AO_(2x), AO_(2y) eachusing an illumination light of different wavelength .[.(λ_(2x)).].(.Iadd.λ₂ .Iaddend.). The mark Au to be detected by the alignment systemAO_(2x) is used for alignment in the X direction, whereas the mark Al tobe detected by the alignment system AO_(2y) is used for alignment in theY direction. These marks Au and Al are formed midway one side extendingin the X direction of the pattern area PA and another side extending inthe Y direction thereof, respectively. The different wavelength TTRalignment systems AO_(2x), AO_(2y) are each able to move an alignmentposition in the X and Y directions. Further, at a location outside thepattern area PA on the reticle R, but within the view field of theprojection lens PL, there is formed a mark RMr to be detected by a TTRalignment system AO₁ using an illumination light of exposure wavelengthλ₁. The position of the mark RMr is always fixedly located regardless ofreticles having different sizes of the pattern area PA. Then, theexposure light for uniformly illuminating the pattern area PA during theexposure is set not to be shielded by a distal end portion of the. TTRalignment system AO₁. Accordingly, even in the case where a variety ofreticles having different sizes of the pattern area PA are exchangedfrom one to another, the TTR alignment system AO₁ is not required to bemoved and can be held fixed on the apparatus.

Meanwhile, below the projection lens PL, there is located a fiducial(reference mark) plate FP fixedly disposed on wafer stage. The surfaceof the fiducial plate FP is conjugate to the reticle R with respect tothe projection lens PL under the exposure wavelength. A fiducial markFMr to be detected at the same time as the mark RMr on the reticle R andfiducial marks Fu, Fl to be detected at the same time as the marks Au,Al on the reticle R, respectively, are formed on the surface of thefiducial plate FP by etching of a chromium layer, etc. The mark Fucorresponds to the mark Au and is formed in a wide area enough to coverany positions to which the mark Au is movable on the reticle R. Thisalso equally applies to the mark Fl.

Then, in the configuration of FIG. 1, when the fiducial plate FP ispositioned such that the mark RMr (the third mark area) and the fiducialmark FMr (the fourth mark area) are simultaneously detected by the TTRalignment system AO₁, the mark Au (the first mark area) and the fiducialmark Fu (the second mark area) are simultaneously detected by the TTRalignment system AO₁, for example. Since the positional relationshipbetween the fiducial marks FMr and Fu on the fiducial plate FP, as wellas the positional relationship between the marks RMr and Au on thereticle R are precisely known in advance, a difference between theamount or alignment error detected by the different wavelength TTRalignment system AO_(2X) and the amount of alignment error detected bythe exposure wavelength TTR alignment system AO₁ represents an offsetamount in the X direction attributable to chromatic aberration at theposition of the mark Au.

By storing this offset amount, therefore, the error can be easilycorrected when the reticle R and the wafer are later aligned with eachother by using the TTR alignment system AO_(2X).

In this way, with the different wavelength TTR alignment system and theexposure wavelength TTR alignment system being arranged independently ofeach other, but to carry out the simultaneous mark detecting operation,an influence of distortion can be corrected more strictly and reliably.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a configuration for explaining theprinciples of a stepper according to an embodiment of the presentinvention;

FIG. 2 is a view showing a detailed configuration of the stepperaccording to the embodiment of the present invention:

FIGS. 3 and 4 are views for explaining an optical system of a differentwavelength TTR alignment system;

FIG. 5 is a plan view showing an arrangement of reticle grating marks;

FIG. 6 is a plan view showing an arrangement of a wafer grating marks;

FIG. 7A is a plan view showing a fiducial grating for a reticle;

FIG. 7B is a plan view showing a field iris for a wafer;

FIG. 8 is a view for explaining an optical system of the differentwavelength TTR alignment system;

FIG. 9 is a perspective view showing a practical arrangement of thedifferent wavelength TTR alignment system and an exposure wavelength TTRalignment system;

FIG. 10 is a plan view showing a practical pattern arrangement of thereticle;

FIG. 11 is a plan view showing a pattern arrangement on a fiducial markplate; and

FIGS. 12A to 12C are views showing distortion characteristics at thewavelength of an exposure light and a different wavelength.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a configuration of a stepper according to one preferredembodiment of the present invention will be described with reference toFIGS. 2 to 11.

While this embodiment employs a different wavelength TTR alignmentsystem of two-beam interference type using gratings, as disclosed in theabove cited U.S. application Ser. No. 192,784, the present invention isalso applicable to other types of alignment system (including an imagedetecting method or spot scanning method) exactly in the same manner.

In FIG. 2, a dichroic mirror DM is obliquely disposed above a reticle Rat an angle of 45° to bend an optical axis AX of a projection lens PL ata right angle or horizontally on the drawing. The dichroic mirror DMreflects an exposure light propagating along the optical axis AX from anillumination system for exposure (not shown) at a percentage greaterthan about 90% so that the reflected light is directed toward a patternarea PA on a reticle R. The reticle R is held on a reticle stage RSwhich is finely movable in the two dimensional directions (X, Y, θdirections), the reticle stage RS being positioned by a driver 1. On theother hand, an XY stage ST for holding a wafer W and a fiducial plate FPthereon is disposed below the projection lens PL and movable in the Xand Y directions by a motor 2, the coordinate position of the stage STbeing successively measured by a laser interferometer 3. Positioning ofthe stage ST is performed by a stage driver circuit 4 which monitors themeasured value of the interferometer 3 and drives the motor 2. The stagedriver circuit 4 controls the movement and positioning of the stage STbased, on a command from a main controller 5. The main controller 5 alsosupervises control of the driver 1.

While a different wavelength TTR alignment system of the this embodimentis constructed above the dichroic mirror DM in two to four eyes(representing the number of object lenses), FIG. 2 shows only one eye,i.e., the system corresponding to AO_(2X) in FIG. 1.

A tele-centric object lens OBJu and a mirror M₁ are held by a holder 11which is movable in the X and Y directions by the driver 10 togetherwith the object lens and the mirror.

The object lens OBJu is arranged to be out of interference with thedichroic mirror DM and to have its optical axis vertical to the reticleR.

An illumination light for the different wavelength alignment is emittedin the form of a linearly polarized beam LB from a laser light source 12such as a helium-neon or argon ion laser, and enters a two-beam formingfrequency shifter unit 13. In the unit 13, there are disposed means fordividing the beam LB into two beams, two AOMs (acousto-optic modulators)130A, 130B for respectively applying high-frequency modulations(frequency shifts) to the divided two beams, small lenses 131A, 131B forfocusing the respective frequency shifted beams, and so on. The AOM 130Ais driven at frequency f₁ (for example, 80 MHz) and a 1st orderdiffracted beam LB₁ therefrom is taken out through the small lens 131A,whereas the AOM 130B is driven at frequency f₂ (for example, 80.03 MHz)and a 1st order diffracted beam LB₂ therefrom is taken out through thesmall lens 131B. The two beams LB₁, LB₂ have their principal rays setparallel to the optical axis of the alignment system and symmetricalabout the optical axis, and are each divided by a beam splitter 14 intotwo beams. A pair of divided beams L_(r1), L_(r2) having passed throughthe beam splitter 14 are converted by condenser lens (inverse Fouriertransform lens) 15 into fluxes of parallel light which cross each otherin its focus plane on the rear or downstream side. A reference grating16 is disposed in the rear focus plane so as to produce an interferencefringe with the pitch depending on the crossing angle and wavelength ofthe two beams L_(r1), L_(r2). The interference fringe moves over thereference grating 16 in one direction at a speed depending on afrequency difference Δf (30 KHz) between the two beams L_(r1) andL_(r2). On the reference grating 16, there is provided a transmissiontype diffraction grating with the pitch being equal to the pitch of theinterference fringe. Accordingly, the reference grating 16 produces aninterference light DR₁ between a 0th order light of the beam L_(r1) anda 1st order light created from the beam L_(r2), as well as aninterference light DR₂ between a 0th order light of the beam L_(r2) anda 1st order light created from the beam L_(r1), the amounts of theinterference lights DR₁, DR₂ being detected by a photoelectric element17. Here, the intensity of the interference lights DR₁, DR₂ is changedsinusoidally at the frequency difference Δf (30 KHz), i.e., beatfrequency, so that an output signal SR from the photoelectric element 17becomes an AC signal of 30 KHz. This signal SR is used as a referencesignal for phase comparison in the alignment process.

FIG. 3 is a view showing optical paths of the beams LB₁, LB₂ andbehaviors thereof in detail from the lens 15 to the photoelectricelement 17. First, the two beams LB₁, LB₂ are respectively condensed bythe action of the small lenses 131A, 131B in a plane EPa to become beamwaists. The plane EPa is a Fourier plane in coincidence with a focusplane of the lens 15 on the front or upstream side, so that the beamsL_(r1), L_(r2) propagate from the lens 15 in the form of parallel lightfluxes to cross each other on the reference grating 16. Respectivepositions of the beams L_(r1), L_(r2) in the plane EPa are set to besymmetrical about the optical axis AXa of the alignment system, and thedistance of each beam position from the optical axis AXa determines anincident angle θ of the beams L_(r1), L_(r2) with respect to thereference grating 16.

Following the general analytical theory, a diffraction angle α of ±1storder diffracted lights with respect to a 0th order light, produced by acoherent beam of wavelength λ impinging upon a one-dimensional gratingwith a pitch Pg, is uniquely determined by sin α=λ/Pg. Therefore, bysetting the pitch Pg of the reference grating 16 such that thediffraction angle α of the 1st order diffracted light becomes just 2θwith respect to the 0th order light of the beam L_(r1) (i.e., theinterference light DR₁), the 1st order diffracted light of the beamL_(r1) produces coaxially with the 0th order light of the beam L_(r2)(i.e., the interference light DR₂).

At this time, the pitch of the interference fringe produced on thereference grating 16 is equal to the grating pitch Pg.

The photoelectric element 17 has light receiving surfaces 170A, 170B forseparately receiving the two interference beams DR₁, DR₂. Respectivephotoelectric signals from the light receiving surfaces 170A, 170B areadded to each other by an adder 171 in an analog manner and theresulting sum is outputted as the reference signal SR.

Returning again to the explanation of FIG. 2, two beams L_(m1), L_(m2)reflected by the beam splitter 14 are further reflected by another beamsplitter 18 to enter the object lens OBJu through the mirror M₁. Theobject lens OBJu converts the two beams L_(m1), L_(m2) into respectivefluxes of parallel light which cross each other in a spatial plane IP.The plane IP is spaced from the reticle R by a distance corresponding tothe amount ΔL of axial chromatic aberration in the direction of theoptical axis AX, and is conjugate to the wafer W or the fiducial plateFP under the wavelength (λ₂) of the illumination beam LB for alignment.The two beams L_(m1), L_(m2) having crossed each other in the plane IPare spaced from each other in a mark area Au on the reticle R and, afterbecoming beam waists in a pupil EP of the projection lens PL, they crosseach other again on the fiducial plate FP (or the wafer W) in the formof parallel light fluxes.

FIG. 4 is a view of optical paths showing behavior of the beams L_(m1),L_(m2). The focus plane of the object lens OBJu on the front or upstreamside is coincident with the plane EPa shown in FIG. 3, so that the twobeams L_(m1), L_(m2) turn to beam waists in the plane EPa. The two beamsL_(m1), L_(m2) exiting from the object lens OBJu become fluxes ofparallel light and cross each other in the plane IP, i.e., in the rearfocal plane of the object lens OBJu. Thereafter, the beam L_(m1)illuminates a reticle grating mark Aua in the mark area Au on thereticle R and also passes through a transparent portion of the mark areaAu, followed by entering the projection lens PL. Then, the beam L_(m1)becomes a flux of parallel light again which obliquely illuminates agrating in a mark area Fu on the fiducial plate FP (or the wafer W).

Likewise, the beam L_(m2) illuminates a reticle grating mark Aub in themark area Au on the reticle R and also passes through a transparentportion of the mark area Au, following which the beam L_(m2) obliquelyilluminates the grating in the mark area Fu on the fiducial plate FP (orthe wafer W) in a direction symmetrical to the beam L_(m1).

As shown in FIG. 4, the two beams L_(m1), L_(m2) become fluxes ofconverging light in a pupil space (comprising the optical path betweenthe object lens OBJu and the small lenses 131A, 131B and the interior ofthe projection lens PL) with their principal rays being parallel to theoptical axis AXa, and then become fluxes of parallel light in an imagespace (comprising the optical path between the object lens OBJu and theprojection lens PL and the optical path between the projection lens PLand the fiducial plate FP or the wafer W) with their principal rayscrossing each other in a focal plane (image plane) of the projectionlens PL.

From the mark area Fu on the fiducial plate FP, an interference light BTis vertically produced in the upward direction. The interference lightBT is resulted from interference between a 1st order diffracted lightvertically produced from the mark Fu upon irradiation of the beam L_(m1)and a 1st order diffracted light vertically produced from the mark Fuupon irradiation of the beam L_(m2), the interference light BT being aflux of parallel light in the image space. After converging into, a beamwaist at the center of the pupil EP of the projection lens PL, theinterference light BT reversely propagates along the optical axis Axa ofthe object lens OBJu while passing through a transparent portion(between the marks Aua and Aub) at the center of the mark area on thereticle R in the form of a parallel light flux. The interference lightBT converges again into a beam waist at the center of the front focalplane of the object lens PL, that is, the plane EPa conjugate to thepupil EP of the projection lens PL, followed by returning toward a lightreceiving system while passing through the mirror M₁ and the beamsplitter 18 in this order as shown in FIG. 2. Structure of the mark areaAu on the reticle R will now be explained with reference to FIGS. 5 and6. As shown in FIG. 5, a light shielding band ESB of constant width isformed all around the circuit pattern area PA on the reticle R. At apart of the light shielding band ESB, there are arranged two gratingmarks Aua, Aub with a light shielding portion ESB' therebetween. Themarks Aua and Aub have the same pitch, and their width in a directionperpendicular to the pitch direction is set about a half the total widthof the transparent window. The width of the light shielding portion ESB'is also set about a half the total width of the transparent window. Agrating mark WMu on the wafer W is located in a portion WMu' in the markarea under the wavelength for alignment. The two beams L_(m1), L_(m2)respectively illuminate the grating marks Aua and Aub in a rectangularshape. Meanwhile, as shown in FIG. 6, the beam L_(m1) having passedthrough the transparent portion adjacent to the mark Aua on the sameside as the pattern area PA illuminates the wafer mark WMu, and the beamL_(m2) having passed through the transparent portion adjacent to themark Aub on the same side as the pattern area PA also illuminates thewafer mark WMu. The interference light BT vertically produced from themark WMu returns to the object lens OBJu after passing through theportion WMu' in the plane of the reticle R as shown in FIG. 6.

When the beams L_(m1), L_(m2) respectively illuminate the reticlegrating marks Aua, Aub in FIG. 4, higher order diffracted lights arealso reflected along with 0th order lights D₀₁,D₀₂. In this embodiment,the pitch of the mark Aua (Aub) is set so that assuming an incidentangle of the beam L_(m1) (L_(m2)) upon the grating mark Aua (Aub) be θ',the diffraction angle of the 1st order diffracted light with respect tothe 0th order light D₀₁ (D₀₂) becomes exactly 2θ'. With such setting,the grating mark Aua produces a 1st order diffracted light D₁₁ (with thefrequency shift f₁) propagating along the optical path of the beamL_(m1) exactly in a reversed direction, whereas the grating mark Aubproduces a 1st order diffracted light D₁₂ (with the frequency shift f₂)propagating along the optical path of the beam L_(m2) exactly in areversed direction. Accordingly, along with the interference light BT,the 1st order diffracted beams D₁₁, D₁₂ are also returned toward thelight receiving system through the object lens OBJu. Referring to FIG. 2again, the interference light BT and the 1st order diffracted lightsD₁₁, D₁₂ pass through the beam splitter 18 and enter a condensing lenssystem 19. The lens system 19 is an inverse Fourier transform lens whichconverts all of the interference light BT and the 1st order diffractedlights D₁₁, D₁₂ into fluxes of parallel light and causes them to crosseach other in a focal plane (image conjugate plane) of the lens system19. The three beams having passed through the lens system 19 are eachdivided into two parts by a beam splitter 20. Respective parts havingpassed through the beam splitter 20 reach a fiducial grating plate 21for the reticle, whereas the reflected parts reach a field iris 22. Thefiducial grating plate 21 and the field iris 22 are both conjugate tothe .[.plate.]. .Iadd.plane .Iaddend.IP (i.e., the plane conjugate tothe fiducial plate FP or the wafer W). Accordingly, at the fiducialgrating plate 21, the 1st order diffracted lights D₁₁, D₁₂ cross eachother to produce an interference fringe in the crossed area. Theresulting interference fringe of course flows or driftsone-dimensionally at the beat frequency Δf (30 KHz). As shown in FIG.7A, therefore, a transmission type diffraction grating 21A is providedon the fiducial grating plate 21 covered with a chromium layer, and aninterference light BTr between two diffracted lights .Iadd.is.Iaddend.coaxially produced from the grating 21A. This behavior will nowbe described by referring to FIG. 8. Because of meeting an imageconjugate relation, the fiducial grating plate 21 is conjugate to theplane IP and the wafer mark WMu (or the surface of the fiducial plateFP) so that the interference light BT also impinges upon the fiducialgrating plate 21 in addition to the 1st order diffracted lights D₁₁,D₁₂. However, since the wafer mark WMu and the reticle marks Aua, Aubare arranged in the X-Y plane to be shifted laterally, the interferencelight BT returns to a portion (light shielding portion) 21B on thefiducial grating plate 21, which portion is adjacent to the grating 21Awhere the two 1st order diffracted lights D₁₁, D₁₂ cross each other, asshown in FIG. 7A.

Accordingly, by just setting the position and size of the grating 21A onthe fiducial grating plate 21 in match with the size of the mark Aua orAub, the interference light BT from the wafer mark WMu can be shielded.

Respective 0th order lights BTo of the 1st order diffracted lights D₁₁,D₁₂ illuminating the grating 21A on the fiducial grating plate 21propagate so as to deviate from a photoelectric element 23, and only theinterference light BTr between two 1st order diffracted lightsvertically produced from the grating 21A is received by thephotoelectric element 23. This technique is the same as the case oftaking out the interference light BT from the wafer mark WMu as shown inFIG. 4. In either case, the grating pitch of the wafer mark WMu or thegrating 21A is set exactly two times the pitch of the interferencefringe produced thereon.

The intensity of the interference light BTr thus received by thephotoelectric element 23 is changed sinusoidally at the beat frequencyΔf (30 KHz). Therefore, an output signal Sm of the photoelectric element23 becomes an AC signal which is linearly changed in a phase differencerelative to the reference signal SR depending on the amount ofdisplacement of the reticle marks Aua, Aub in the pitch direction on thebasis of the reference grating 16.

On the other hand, the interference light BT and the 1st orderdiffracted lights D₁₁, D₁₂, all reflected by the beam splitter 20 shownin FIG. 2, reaches the field iris 22. The iris 22 is formed with anopening 22A which allows only the interference light BT to passtherethrough, as shown in FIG. 7B, while the two 1st order diffractedlights D₁₁, D₁₂ are shielded by a light shielding portion 22B.

The interference light BT having passed through the iris 22 reaches aphotoelectric element 26 through a mirror 24 and a condenser lens 25.The photoelectric element 26 produces an output signal Sw depending onchange in the intensity of the interference light BT. This output signalSw also becomes an AC signal of which level is changed sinusoidally atthe beat frequency Δf, the phase of the AC signal relative to thereference signal SR being changed in proportion to the amount ofdeviation of the mark WMu on the wafer W or the fiducial mark Fu on thefiducial plate FP from the reference grating 16.

The reference signal SR and the output signals Sm, Sw are inputted to aphase difference measurement unit 27 which determines a phase differenceφm of the output signal Sm relative to the reference signal SR,determines a phase difference φw of the output signal Sw relative to thereference signal SR, and further a difference therebetween Δφ=φm-φw. Inthe case of this embodiment, since the diffraction grating pitch on thefiducial plate FP (or the wafer W) is two times the pitch of theinterference fringe produced thereon, one period (±180°) of the phasedifference Δφ corresponds to 1/2 (±1/4 pitch) of the diffraction gratingpitch. Based on the phase difference Δφ, the measurement unit 27calculates position correction amounts (or position shift amounts) ΔX,ΔY of the water stage ST or the reticle stage RS, values of thoseamounts being delivered to the main controller 5.

In the foregoing, the system from the driver 10 to the measurement unit27, including the object lens OBJu, corresponds to first mark detectingmeans in the present invention.

Meanwhile, in FIG. 2, the mark RMr on the reticle R is detected by a TTRalignment system for exposure (corresponding to AO₁ in FIG. 1) whichcomprises a mirror M₂, an object lens OBr, a beam splitter 30, a lenssystem 31, an illumination field iris 32, a condenser lens 33, a fiber34, a focusing lens 35, a beam splitter 36, and CCD image sensors 37A,37B. The fiber 34 emits an illumination light at the exposure wavelengthto uniformly irradiate the iris 32 through the condenser lens 33. Theillumination light having passed through an opening of the iris 32enters the object lens OBr through the lens system 31 and the beamsplitter 30, following which it is bent by the mirror M₂ at a rightangle to vertically illuminate a local area of the reticle R includingthe mark RMr downwards. The iris 32 is conjugate to the reticle R sothat an opening image or the iris 32 is focused on the reticle R. Thebeam splitter 30 is located near a front focus plane or the tele-centricobject lens OBr, i.e., a plane conjugate to the pupil EP of theprojection lens FL, thereby reflecting a part of the light returned fromthe object lens OBr toward the focusing lens 35. The CCD image sensors37A, 37B have their light receiving surfaces which are conjugate to thereticle R through the object lens OBr and the focusing lens 35, and alsoconjugate to the wafer W or the fiducial plate FP through the projectionlens PL. Incidentally, an exit end of the fiber 34 is located to beconjugate to the pupil EP of the projection lens PL for achievement ofthe Keller's illumination. In the case of this embodiment, the opticalpath between the object lens OBr and the beam splitter 30 exhibits anafocal system and, therefore, a deviation in the position of the markRMr can be compensated for by arranging the object lens OBr and themirror M₂ integrally with each other in such a manner as able to movetogether horizontally in FIG. 2. But, in consideration of stability ofthe system, the mirror M₂ is here fixedly located at a position outsidethe pattern area PA having the maximum dimension possibly expected onthe reticle R.

CCD image sensors 37A, 37B are arranged to have an angular spacing of90° therebetween so that their horizontal scan lines coincide with the Xand Y directions, respectively, for making position measurements of thecrossshaped mark RMr in the X and Y directions separately. The reason isto avoid a difference in resolution as developed when using a single CCDimage sensor to detect shifts of the mark image in both the horizontaland vertical directions, because a usual CCD image sensor .[.have.]..Iadd.has .Iaddend.different degrees of pixel resolution in thehorizontal and vertical directions. An image processing unit 38 receivesrespective image signals (video signals) from the CCD image sensors 37A,37B, detects a position shift amount of the mark RMr on the reticle Rfrom the fiducial mark FMr on the fiducial plate FP, and then.[.deliver.]. .Iadd.delivers .Iaddend.information of the position shiftamount to the main controller 5.

In addition to the above configuration, there are also provided a globalmark detection system 40 of off-axis type for detecting a globalaligment mark on the wafer W, a latent image in a resist layer or therespective fiducial marks on the fiducial plate FP, and a processingunit 42 for processing a signal from the system 40. Furthermore,adjustment units 50A, 50B for adjusting or correcting various focusingcharacteristics of the projection lens PL are provided and supervisedunder control of the main controller 5. The adjustment unit 50A has afunction of changing magnification, focus position, distortion, etc. ofthe projection lens PL by, for example, controlling pressure of apredetermined air chamber in the projection lens PL. The adjustment unit50B has a function of finely moving in the axial direction or tilting alens element as one component of the projection lens PL (for example, afield lens on the reticle side).

The different wavelength TTR alignment system and the exposure light TTRalignment system are preferably arranged as schematically shown in FIG.9. As will be seen from FIG. 9, reticle grating mark areas Au, Al, Ar,Ad, each being arranged as shown in FIG. 5, are formed in the respectivesides of the light shielding band ESB around the pattern area PA on thereticle R. The mark areas Au, Ad are used for alignment in the Xdirection plotted in FIG. 9, whereas the mark areas Ar, Al are used foralignment in the Y direction.

Outside the light shielding band ESB, there is also provided a reticlemark RMl similar to the reticle mark RMr at a symmetrical position.

Therefore, object lenses OBr, OBl, of two exposure light TTR alignmentsystems are arranged to respectively detect the marks RMr, RMl below thedichroic mirror DM, and object lenses OBJu, OBJd, OBJr, OBJl of fourdifferent wavelength TTR alignment systems are arranged to respectivelydetect the mark areas Au, Ad, Ar, Al through the dichroic mirror DM.

When the reticle R and the wafer W are actually aligned with each otheron a die-by-die basis by using the different wavelength TTR alignmentsystems, the four eyes are not always required to be usedsimultaneously, but three or two eyes may be used instead. This isrelied on the fact that even in the case where any of correspondingmarks (WMu, WMd, WMr, WMl) in one shot area on the wafer is defective,if one eye in the X direction and one eye in the Y direction at minimumproperly output photoelectric signals, an alignment error (interruptionof the sequence) can be avoided to the utmost by executing the alignmentfor the shot area of interest.

The mark arrangement on the reticle R as shown in FIG. 9 and the markarrangement on the fiducial plate FP suitable for this embodiment willbe next described with reference to FIGS. 10 and 11.

FIG. 10 shows one preferred example of a pattern layout on the reticleR, in which the pattern area PA having the maximum dimension projectableby the projection lens PL is supposed. The cross-shaped reticle marksRMr, .[.RM.]. .Iadd.RMl .Iaddend.are provided on a line passing thecenter Rcc of the pattern area PA on the reticle R and extendingparallel to the X axis. These marks RMr, RMl are set substantially atthe respective centers of the view fields of the object lenses OBr, OBl.In the reticle R shown in FIG. 10, the four mark areas Au, Ad, Ar, Alare each located at a position farthest from the reticle center Rcc.Stated otherwise, rectangular areas Su, Sd, Sr, Sl indicated by brokenlines in FIG. 10 respectively .[.stand for one.]. .Iadd.are.Iaddend.examples of ranges where the optical axes (detection centers)of the object lenses OBJu, OBJd, OBJr, OBJl of the four differentwavelength TTR alignment systems are movable. Then, in the case of FIG.10, the mark areas Au, Ad, Ar, Al are each provided at the outermostposition of the movable range of the corresponding object lens.

FIG. 10 is illustrated by way of example and, depending on cases, themovable ranges Sr, Sl of the object lenses OBJr, OBJl may protrude intoa region between the movable ranges Su, Sd of the object lenses OBJu,OBJd from the right and left.

FIG. 11 shows the mark arrangement on the fiducial plate FP with doublecross-shaped fiducial marks FMr, FMl provided on the left and rightsides of the center in the X direction. The center-to-center distancebetween the two fiducial marks FMr and FMl is set equal to a valueresulted from multiplying the center-to-center distance between the twomarks RMr and RMl on the reticle by the projection magnification (l/M).Accordingly, when the center Fcc of the fiducial plate FP is madecoincident with the reticle center Rcc, the fiducial mark FMr and thereticle mark RMr are simultaneously observed by the object lens OBr ofone exposure light TTR alignment system under a condition that thereticle mark RMr is positioned between the double lines of the fiducialmark FMr, and the fiducial mark .[.FM.]. .Iadd.FMl .Iaddend.and thereticle mark RMl are simultaneously observed by the object lens OBl ofthe other exposure light TTR alignment system under a condition that thereticle mark RMl is positioned between the double lines of the fiducialmark FMl.

On the fiducial plate FP, there are also provided fiducial mark areasFu, Fd, Fr, Fl in which diffraction gratings are engraved in positionsand sizes respectively corresponding to the movable ranges Su, Sd, Sr,Sl on the reticle R when the center Fcc of the fiducial plate FP is madecoincident with the reticle center Rcc. In the fiducial mark area Fu,for example, a group of grating lines engraved in the X direction withthe constant pitch are formed similarly to the mark WMu on the wafer Wand used for detecting a relative position shift in the X direction fromthe mark area Au on the reticle R. This equally applies to the otherfiducial mark areas Fd, Fr, Fl. Accordingly, even if the size of thepattern area PA (or the light shielding band ESB) is changed uponreplacement of the reticle R with another one, it is possible tosimultaneously detect an X-directional shift of the mark area Au fromthe fiducial mark Fu, an X-directional shift of the mark area Ad fromthe fiducial mark Fd, a Y-directional shift of the mark area Ar from thefiducial mark Fr, and a Y-directional shift of the mark area Al from thefiducial mark Fl by the four eyes (OBJu, OBJd, OBJr, OBJl) so long asthe mark areas Au. Ad, Ar, Al on the reticle are present within themovable ranges Su, Sd, Sr, Sl, respectively.

In FIG. 11, the two groups of plural grating lines making up thefiducial marks Fr and Fl on the left and right sides correspond to eachother in one-to-one relation with respect to the Y direction, whereasthe two groups of plural grating lines making up the fiducial marks Fuand Fd on the upper and lower sides correspond to each other inone-to-one relation with respect to the X direction. Further, thecenter-to-center spacing in the X direction between the mark area Au andAd shown in FIG. 10 is accurately equal to integer times as much as thegrating pitch of the fiducial marks Fu, Fd, whereas the center-to-centerspacing in the Y direction between the mark area Ar and Al is accuratelyequal to integer times as much as the grating pitch of the fiducialmarks Fr, Fl.

Operation of this embodiment, that is, base line measurement of theexposure light TTR alignment system and the different wavelength TTRalignment system in consideration of a distortion, will be describedbelow. At first, an arbitrary reticle (such as shown in FIG. 10, forexample) is set on the reticle stage RS and the reticle alignment isperformed by picking up images of the reticle marks RMr. RMl and themarks FMr, FMl on the fiducial plate FP by the CCD sensor elements 37A,37B of the exposure light TTR alignment systems.

Then, the object lenses OBJu, OBJd, OBJr, OBJl (i.e., the holders 11) ofthe four different wavelength alignment systems are set at positionsrespectively corresponding to the mark areas Au, Ad, Ar, Al, followed bychecking shifts in illumination position of the two beams L_(m1), L_(m2)in the X and Y directions and a tele-centric error of the two beamsL_(m1), L_(m2) by the use of the fiducial marks Fu, Fd, Fr, Fl on thefiducial plate FP. After completion of the checking, the four differentwavelength alignment systems each determines amounts of relativeposition shifts between the reticle R and the fiducial plate FP at thatposition. In other words, the shift amounts in the X direction betweenthe fiducial mark Fu (and Fd) and the reticle mark Au (and Ad), as wellas the shift amounts in the Y direction between the fiducial mark Fr(and Fl) and the reticle mark Ar (and Al) are detected through therespective measurement units 27.

Based on the detected shift amounts, the main controller 5 controls thedriver 1 and the stage driver 4 for servo-driving the reticle stage RSor the wafer stage ST. Since the different wavelength alignment systemsof this embodiment are each of the heterodyne type allowing successivemeasurement of the relative position shift amount even in a conditionthat the reticle mark and the fiducial mark remain at rest.[..The.]..Iadd., the .Iaddend.measurement units 27 continue successivelyoutputting the information on the relative position shifts (such as inthe X, Y and rotating directions). Therefore, the alignment between thereticle R and the fiducial plate FP is continued so that all the phasedifferences Δφ measured by the measurement units 27 of the fourdifferent wavelength alignment systems become zero (or a fixed value).During the above step of the different wavelength alignment, the imageprocessing units 38 of the exposure light TTR alignment systems continuesuccessively (at certain intervals of time) outputting the positionshift amounts (ΔXr, ΔYr) in the X, Y directions between the reticle markRMr and the fiducial mark FMr, as well as the position shift amounts(ΔXl, ΔYl) in the X, Y directions between the reticle mark RMl and thefiducial mark FMl. It is to be noted that in the case of a largedistortion, the corresponding shift amount is added as an offset amountto a design value beforehand. Thus, the shift amounts (ΔXr, ΔYr), (ΔXl,ΔYl) determined by the exposure light TTR alignment systems correspondto averages of distortion differences at the alignment positions (i.e.,the marks Au, Ad, Ar, Al) between the wavelength of the exposure lightand the different wavelength therefrom. To put it in more detail, whenthe results of detecting two pairs of the reticle mark Au (the fiducialmark Fu) and the reticle mark Ad (the fiducial mark Fd) show that thephase differences Δφ therebetween are both zero, the shift amounts (ΔXr,ΔYr), (ΔXl, ΔYl) detected by the image processing units 38 are storedplural times, following which the stored results area averaged todetermine overall alignment errors (in X, Y and θ directions) caused bya distortion difference between the reticle R and the fiducial plate FP.In the case of this embodiment, the reticle R and the fiducial plate FPare simultaneously aligned with each other by using four eyes of thedifferent wavelength TTR alignment systems.[., the.]..Iadd.. The.Iaddend.reticle R and the fiducial plate FP exhibit, as a consequenceof the alignment, slight errors in the X, Y and θ directions dependingon distortion characteristics of the projection lens PL at the differentwavelength. These errors can be assumed as fixed offsets so long as thereticle marks Au, Ad, Ar, Al are not changed in their positions.Therefore, by detecting the shift amounts between the marks RMr, RMl andthe fiducial marks FMr, FMl using the exposure light TTR alignmentsystems, the resulting shift amounts give offset amounts in the X, Y andθ directions which include the distortion amounts of the projection lensat the positions of the marks RMr, RMl under the exposure light.

Accordingly, when actually carrying out alignment of the shot area onthe wafer W by the different wavelength TTR alignment systemsthereafter, it is only required to control the reticle stage RS or thewafer stage ST so that the aligned position is shifted by the aboveoffset amounts to reach the true alignment position. The offset amountscaused by the distortion difference in the X, Y and θ (rotating)directions are calculated in the main controller 5 based on informationof the shift amounts from the image processing unit 38, and stored untilthe reticle R will be realigned or replaced with another one.

As the distortion difference in the projection lens PL between thewavelength of the exposure light and the different wavelength therefrommay be changed upon the adjustment unit 50B being actuated, it isdesirable that immediately after actuating the adjustment unit 50B to alarge extent, the offset amounts are measured again by using thefiducial plate FP.

With this embodiment as stated above, since the mark areas Au, Ad, Ar,Al for the different wavelength TTR alignment are respectively providedon the four sides of the reticle R and the corresponding fiducial marksFu, Fd, Fr, Fl on the fiducial plate FP are simultaneously detected, itis possible to precisely determine the overall alignment errors throughTTR which are caused by an influence of distortions at the differentwavelength. Further, with this embodiment, since the differentwavelength TTR alignment systems and the exposure light TTR alignmentsystems are simultaneously operated through the projection lens PL andthe values measured by the interferometer 3 on the wafer stage ST arenot used at all, errors due to fluctuating air in the atmosphere, whichwould raise a problem associated with the interferometer 3, will not beinvolved so that the offset amounts can be measured with very highprecision.

In addition, since this embodiment employs the different wavelength TTRalignment systems of heterodyne type having extremely high resolution,the highly accurate measurement is achieved. For example, assuming thatthe pitch of the diffraction gratings on the fiducial plate FP is on theorder of 4 μm, the phase difference detectable range (±180°) is given ±1μm. Also, assuming that the practical phase measurement resolution is±2° in consideration of noises and so on, the position shift detectingresolution becomes as high as about ±0.01 μm.

Consequently, with such an arrangement that the reticle R and thefiducial plate FP are subjected to alignment servo control by using thedifferent wavelength TTR alignment systems of heterodyne type, thehighly stable positioning can be achieved.

While the projection lens PL of this embodiment has been explained asbeing tele-centric on both sides thereof, it may of course betele-centric on either side only. In the case of a projection lens beingtele-centric on both sides, the optical axis of the object lens of theexposure light TTR alignment system is vertical to the reticle surfaceand also coincident with the principal ray passing the pupil center ofthe projection lens PL. Therefore, if the mark patterns on the reticleare formed of reflective chromium layers, the light regularly reflectedby the patterns are so strongly detected by the CCD image sensors thatboth the reticle mark RMr, RMl and the fiducial mark FMr, FMl may appearbright. In the case where the fiducial plate FP made of quartz glass orthe like is entirely covered with a chromium layer and the fiducialmarks FMr, FMl are formed by removing the chromium layer by etching orthe like into desired patterns, it may happen that the fiducial marksFMr, FMl look black, but the whole of the surroundings become bright,thereby greatly lowering the contrast of the reticle marks RMr, RMl. Inthis case, an aperture iris (spatial filter) having a ring-shapedopening may be disposed in the illumination optical path of eachexposure light TTR alignment system, e.g., in the pupil conjugate planebetween the beam splitter 30 and the lens system 31 in FIG. 2, forilluminating the reticle R in the dark field. With this arrangement,dark field images are focused on the CCD image sensors such that onlyrespective edges of the reticle marks RMr, RMl and the fiducial marksFMr, FMl glint brightly. Moreover, the spatial filter disposed in thepupil conjugate plane may be formed of a liquid crystal, electrochromic(EC) or the like in which multiple ring-like openings arc patterned inconcentric relation, allowing the illumination to be switched betweenthe dark field and the light field. It is also possible to change thenumber of aperture for the illumination light.

A manner of improving superposition accuracy when using the apparatus asshown in FIG. 2, in particular, a manner of improving the matchingbetween different units of the apparatus, will be described below.

FIG. 12A exaggeratedly represents distortion characteristics (brokenlines) of the projection lens PL under the exposure wavelength anddistortion characteristics (one-dot-chain lines) thereof under thedifferent wavelength (i.e., the wavelength of the alignment light) onthe basis of an ideal lattice (solid lines).

A distortion map like that can be drawn by, for example, making a trialprint using a test reticle. Note that the distortion map under thedifferent wavelength cannot be obtained by the trial printing, but canbe obtained by using the method similar to the above stated embodimentin a combined manner.

At first, a test reticle having vernier marks for superpositionmeasurement at respective ideal lattice points in the pattern area isprepared and aligned by the exposure light TTR alignment system. Then, areticle blind in the exposure illumination system is fully opened andthe test reticle is exposed onto a dummy wafer (such as a photosensitiveresist layer, photo-chromic layer or bare silicon wafer coated with anopto-magnetic medium).

Next, the reticle blind is narrowed so as to illuminate only the verniermark provided at the center of the test reticle. Following that, whilestepping the wafer stage ST at the pitch of the ideal lattice points, anexposure is made in superposed relation to each latent image of thevernier mark on the test reticle having been exposed in advance on astep-by-step basis.

In this case, on assumptions that the stepping of the wafer stage ST iscoincident with the division pitch of the ideal lattice, the distortioncharacteristics under the exposure wavelength on the basis of the ideallattice can be determined by measuring the superposition accuracybetween the latent image of each vernier mark exposed at the first timeand the latent image of the same vernier mark printed at the second timein superposing relation. In the above measurement, the latent images ofthe vernier marks on the dummy wafer may be detected by the global markdetection system 40 of off-axis type shown in FIG. 2, or by the exposurelight TTR alignment system after properly modifying a shape of thevernier mark. Where the dummy wafer is formed of a usual photoresistlayer, the measurement may be performed by the different wavelength TTRalignment system, etc. after once developing the dummy wafer to buildresist images of the vernier marks.

Then, amounts of superposition errors are determined at each of theideal lattice points in that way, and the resulting error amounts arestatistically processed by the method of least squares for determiningthe respective offsets in the X, Y and θ directions. Based on thoseoffsets, the shift amounts (ΔOF_(x1), ΔOF_(y1)), (ΔOF_(x2), ΔOF_(y2)) ofthe images RMr', RMl' of the reticle marks RMr, RMl under the exposurewavelength, indicated by broken lines in FIGS. 12B and 12C, from theideal positions (solid lines) can be presumed as system offsets.

Consequently, when aligning the reticle marks RMr, RMl by the exposurelight TTR alignment systems, it is possible to always make an exposureon the distortion map with the least errors from the ideal lattice, bytaking into account the system offsets.

Further, since the difference in distortion between the exposurewavelength and the different wavelength can be determined following theabove first embodiment, the reticle pattern can be exposed in superposedrelation to each shot area on the wafer under a condition closest to theideal lattice, by further taking into account (or compensating for) thedistortion difference, even in the case of die-by-die alignment usingthe different wavelength TTR alignment systems. This implies that thematching between different units of plural steppers jointly constitutinga single semiconductor manufacture line can be achieved on the order ofthe ideal lattice, and hence that the matching accuracy in themanufacture line can be improved.

Since the above embodiment is premised on using the stepper shown inFIG. 2, the detection center of the exposure light TTR alignment systemis fixedly positioned in the view field of the projection lens.Therefore, after determining and compensating for the system offsets(ΔOF_(x1), ΔOF_(y1)), (ΔOF_(x2), ΔOF_(y2)) during the reticle alignmentbeforehand so that the distortion map under the exposure wavelengthbecomes closest to the ideal lattice, as explained in connection withFIGS. 12B and 12C, distortion errors under the different wavelength maybe determined at each alignment position using the different wavelengthTTR alignment system.

According to the present invention, as described above, since adifferent wavelength TTR alignment system and an exposure light TTRalignment system are arranged to be separated from each other and tosimultaneously detect a group of fiducial marks located on the imageplane of a projection optical system, the distortion difference asdeveloped in when using the different wavelength can be preciselymeasured. In other words, since the alignment light at the exposurewavelength and the alignment light at the different wavelength aredetected by the respective TTR alignment systems exactly at the sametiming through the projection optical system, the detected resultscommonly include measurement errors due to minute fluctuations in airflows, temperature distribution, etc. within the optical paths, therebyallowing those measurement errors to be canceled out. Another advantageis in that since the present invention does not rely on the method usinga laser interferometer while running a wafer stage or the like, theresults will not be affected by measurement accuracy (reproducibility)of the laser interferometer itself.

Further, according to the present invention, since a plurality of TTRalignment systems can be simultaneously operated using the group offiducial marks, it is also possible to implement beam positioning,tele-centric checking and focus checking at the same time, which arenecessary upon the an object lens of the TTR alignment system beingmoved, resulting in an advantage of increasing a throughput in settingof the TTR alignment system.

I claim:
 1. A projection optical apparatus comprising:a projectionoptical system for projectively focusing a pattern image of a mask underillumination by a light of first wavelength characteristic onto asensitive substrate, a stage for holding said sensitive substrate, afiducial plate disposed on said stage, first mark detection means forilluminating a light of second wavelength characteristic different fromsaid first wavelength characteristic, through a first mark area formedon said mask and said projection optical system, onto .[.said sensitivesubstrate or.]. a second mark area formed on said fiducial plate, andthen detecting optical information produced from said second mark area,a fourth mark area formed on .[.said sensitive substrate or.]. saidfiducial plate and arranged in a predetermined positional relationshiprelative to said second mark area, a third mark area formed on said maskand arranged in a predetermined positional relationship relative to saidfirst mark area, second mark detection means for illuminating said lightof first wavelength characteristic onto said fourth mark area throughsaid third mark area and said projection optical system, and thendetecting optical information produced from said fourth mark area, undera condition that said first mark detection means is detecting theoptical information produced from said second mark area, and errordetection means for detecting detection errors due to a distortion atrespective positions in the view field of said projection optical systemwhere said first mark area and said second mark area are present, basedon the detected results by said first mark detection means and saidsecond mark detection means.
 2. A projection optical apparatus accordingto claim 1, wherein:said first mark detection means has a first objectlens system movable depending on position change of said first mark areaon said mask, and .[.the.]. .Iadd.a .Iaddend.uniform mark pattern isformed in said second mark area over a movable range of said firstobject lens system.
 3. A projection optical apparatus comprising:aprojection optical system disposed between first and second planes suchthat said first and second planes are optically conjugate to each otherunder a first wavelength characteristic, alignment mark means includingfirst and third mark means disposed on said first plane with apredetermined positional relationship therebetween, and second andfourth mark means disposed on said second plane with a predeterminedpositional relationship therebetween; first mark detection means forilluminating a light of second wavelength characteristic different fromsaid first wavelength characteristic onto said second mark means throughsaid first mark means and said projection optical system, and thendetecting first optical information produced from said second markmeans, second mark detection means for illuminating said light of firstwavelength characteristic onto said fourth mark means through said thirdmark means and said projection optical system, and then detecting secondoptical information produced from said fourth mark means, and means forcreating information of alignment errors developed between said firstand second planes in the directions enough to define a plane, based onsaid first and second optical information. .Iadd.
 4. A projectionexposure apparatus for exposing a pattern area of a mask irradiated withfirst illumination light onto a substrate through a projection opticalsystem, the apparatus comprising:(a) a movable stare supporting thesubstrate and moving in a predetermined plane perpendicular to anoptical axis of said projection optical system; (b) a mask holdersupporting the mask which has a mask mark formed at a peripheralposition of the pattern area; (c) a fiducial plate mounted on saidmovable stage and having first and second fiducial marks on its topface, the first fiducial mark being aligned through said projectionoptical system with the mask mark when said movable stage is located ata predetermined reference position with respect to the image field ofsaid projection optical system; (d) a first alignment system irradiatingthe mask mark and the first fiducial mark with the first illuminationlight and receiving a light from the mask mark and a light from thefirst fiducial mark through said projection optical system, to detect apositional relationship between the mask mark and the first fiducialmark, when said movable stage is; located near the reference position;and (e) a second alignment system including a predetermined detectingreference, irradiating the second fiducial mark with second illuminationlight which has different wavelength from the first illumination lightand receiving a light from the second fiducial mark through saidprojection optical system, to detect a positional relationship betweenthe detecting reference and the second fiducial mark, when said movablestage is located near the reference position..Iaddend..Iadd.5. Aprojection exposure apparatus for exposing a pattern area of a maskirradiated with first illumination light onto a substrate through aprojection optical system, the apparatus comprising: (a) a movable stagesupporting the substrate and moving in a plane perpendicular to anoptical axis of said projection optical system; (b) a fiducial platefixedly mounted on a portion of said movable stage and having first andsecond fiducial patterns on its surface, said first and second fiducialpatterns being arranged with predetermined positional relationship; (c)a first alignment optical system detecting a light from said firstfiducial pattern through said projection optical system and a peripheralportion of the pattern area of said mask and a light from a mask markformed within the peripheral portion of said mask, when said mask markand the first fiducial pattern are irradiated with a light of a samewavelength as the first illumination light; (d) a controller driving andpositioning said movable stage so that the first fiducial pattern issubstantially aligned with the mask mark and simultaneously the secondfiducial pattern is located at a predetermined position in the imagefield of said projection optical system; and (e) a second alignmentoptical system detecting a light from the second fiducial patternthrough said projection optical system, when the second fiducial patternis irradiated with a light having different wavelength than the firstillumination light; wherein said movable stage is stationary until saidfirst and second alignment optical systems complete the light detectingoperations..Iaddend..Iadd.6. A method for examining a variation of thebase line distance between a center point of a reticle and a detectingcenter of an alignment system which detects positional deviation of amark formed on a substrate for aligning the substrate through aprojection optical system, the method comprising the steps of:(a)providing a fiducial plate within the image field of said projectionoptical system instead of the substrate, said fiducial plate having afirst fiducial mark to be aligned with a reticle mark through saidprojection optical system and a second fiducial mark disposed at apredetermined positional relationship with respect to the first fiducialmark; (b) positioning said fiducial plate at a reference position sothat the first fiducial mark and the reticle mark are substantiallyaligned with each other through said projection optical system; and (c)detecting the positional deviation between the detecting center of saidalignment system and the second fiducial mark to determine the variationof said base line distance while said fiducial plate is stationary atthe reference position..Iaddend..Iadd.7. A method for aligning asubstrate and a mask having a circuit pattern, and for exposing a animage of the circuit pattern onto the substrate through a projectionoptical system, the method comprising the steps of:(a) detecting a maskmark formed on the mask and a first fiducial mark disposed on a movablestage through said projection optical system with a first alignmentsystem using a first wavelength of light and, substantially at the sametime, detecting a second fiducial mark disposed on said movable stagethrough said projection optical system with a second alignment systemusing a different wavelength of light, when said movable stage isstationary at a baseline measurement position; (b) determining apositional relationship between a detecting reference of said secondalignment system and the circuit pattern of the mask in an image fieldof said projection optical system based on detection results obtained instep (a); (c) detecting an alignment mark formed on the substrate, whichis mounted on said movable stage, with said second alignment system; (d)determining a positional relationship between the detecting reference ofpaid second alignment system and the alignment mark of the substrate inthe image field of said projection optical system based on a detectionresult obtained in step (c); and (e) positioning said movable stage toalit a shot area of the substrate and the image of the circuit patternbased on the positional relationships determined in steps (b) and (d),and exposing the shot area of the substrate..Iaddend..Iadd.8. A methodaccording to claim 7, wherein, in the step (a), said first alignmentsystem generates a first deviation signal representing a positionalrelationship of the mask mark and the first fiducial mark and,substantially at the game time, said second alignment system generates asecond deviation signal representing a positional relationship of thedetecting reference of said second alignment system and the secondfiducial mark..Iaddend..Iadd.9. A method according to claim 7, whereinthe substrate has first and second X-direction alignment marks and firstand second Y-direction alignment marks, and said second alignment systemincludes a first objective lens for detecting the first X-directionaliment mark, a second objective lens for detecting the firstY-direction alignment mark, a third objective lens for detecting thesecond X-direction alignment mark, and a fourth objective lens fordetecting the second Y-direction alignment mark..Iaddend..Iadd.10. Amethod according to claim 9, wherein the step (c) includes detecting atleast one of said first and second X-direction alignment marks and atleast one of said first and second Y-direction alignment marks, and thestep (d) includes determining positional relationships between thedetecting reference of said alignment system and each of said oneX-direction alignment mark and said one Y-direction alignmentmark..Iaddend..Iadd.11. A method according to claim 7, wherein the steps(a) and (b) are performed after operation of an adjustment unit whichadjusts a magnification or a distortion characteristic of saidprojection optical system..Iaddend..Iadd.12. A projection exposureapparatus comprising:a projection optical system disposed between a maskand a substrate to project on the substrate an illumination lightirradiating the mask; a plate having fiducial marking formed thereon anddisposed in an image field of said projection optical system; a firstalignment system irradiating a portion of said fiducial marking with afirst alignment light different from said illumination light inwavelength to detect optical information produced from said fiducialmarking through said projection optical system; a second alignmentsystem irradiating a portion of said fiducial marking with a secondalignment light having substantially the same wavelength as saidillumination light through said projection optical system to detect apositional relationship between said mask and said plate; and analignment controller connected to said first and second alignmentsystems to detect an offset amount of said first alignment system causedby said projection optical system..Iaddend..Iadd.13. An apparatusaccording to claim 12, whereinsaid fiducial marking includes two markpatterns disposed at a predetermined positional relationship so thatsaid two mark patterns are detected substantially at the same time bysaid first and second alignment systems, respectively..Iaddend..Iadd.14.An apparatus according to claim 12, wherein said first alignment systemincludes a photodetector which receives, through a mark area on saidmask, diffraction light produced from said fiducial marking and passingthrough said projection optical system..Iaddend..Iadd.15. An apparatusaccording to claim 12, further comprising: an adjustment unit connectedto said projection optical system to adjust an optical characteristic ofsaid projection optical system, wherein said alignment controllerdetermines the offset amount of said first alignment system after anadjustment of said optical characteristic..Iaddend..Iadd.16. Aprojection exposure apparatus comprising: a projection optical systemdisposed between a mask and a substrate to project on the substrate anillumination light irradiating the mask; a first alignment systemirradiating a first fiducial mark with a first alignment light differentfrom said illumination light in wavelength to detect optical informationproduced from said first fiducial mark through said projection opticalsystem; a second alignment system irradiating a second fiducial markwith a second alignment light having substantially the same wavelengthas said illumination light through said projection optical system todetect a positional relationship between said second fiducial mark and amark on said mask; and a plate disposed on a substrate side with respectto said projection optical system, said first and second fiducial marksbeing formed on said plate at a predetermined positional relationship sothat said first and second alignment systems respectively detect saidfirst and second fiducial marks substantially at the sametime..Iaddend..Iadd.17. An apparatus according to claim 16, furthercomprising:an alignment controller connected to said first and secondalignment systems to determine a base line amount of said firstalignment system..Iaddend..Iadd.18. A projection exposure method forprotecting an illumination light irradiating a mask through a projectionoptical system on a substrate to transfer a pattern of said mask ontosaid substrate, comprising the steps of: disposing a plate formed with afirst fiducial mark and a second fiducial mark in an image field of saidprojection optical system; detecting said first fiducial mark throughsaid projection optical system by a first alignment system which uses afirst beam different from said illumination light in wavelength;detecting said second fiducial mark through said projection opticalsystem by a second alignment system which uses a second beam havingsubstantially the same wavelength as said illumination light; anddetermining a base line amount of said first alignment system based onoutputs of said first and second alignment systems..Iaddend..Iadd.19. Amethod according to claim 18, wherein said step of detecting said firstfiducial mark by said first alignment system and said step of detectingsaid second fiducial mark by said second alignment system are performedsubstantially at the same time..Iaddend..Iadd.20. A method according toclaim 19, wherein said plate is substantially at rest during said stepsof detecting said first and second fiducial marks..Iaddend..Iadd.21. Amethod according to claim 18, wherein said base line amount of saidfirst alignment system is determined after an adjustment of an opticalcharacteristic of said projection optical system or a position of saidfirst alignment system..Iaddend..Iadd.22. A projection exposure methodfor protecting an illumination light irradiating a mask through aprojection optical system on a substrate to transfer a pattern of saidmask on said substrate, comprising the steps of: disposing a plateformed with fiducial marking in an image field of said projectionoptical system; detecting said fiducial marking through said projectionoptical system by a first alignment system which uses a first beamdifferent from said illumination light in wavelength and by a secondalignment system which uses a second beam having substantially the samewavelength as said illumination light, respectively; and determining abase line amount of said first alignment system based on outputs of saidfirst and second alignment systems..Iaddend.